1. Field of the Invention
The invention relates to a supercharging system and a diagnostic method for a supercharging system.
2. Description of Related Art
As supercharging systems applied to internal combustion engines, supercharging systems such as a turbocharger that uses exhaust energy for supercharging are available. This type of supercharging system includes a compressor and a turbine. Said compressor is mounted in an intake passage of the internal combustion engine, pressurizes intake air flowing through the intake passage, and discharges the pressurized intake air into a combustion chamber of the internal combustion engine. Said turbine is mounted in an exhaust passage of the internal combustion engine and operated by a flow force of exhaust gas flowing through the exhaust passage to drive the compressor. Furthermore, this type of supercharging system that includes a bypass passage and a waste gate valve is also available. In order to actively control a supercharging operation of the supercharging system, said bypass passage is used for the exhaust gas to bypass the turbine and flow therethrough. The waste gate valve blocks a flow of the exhaust gas flowing through the bypass passage when the waste gate valve is fully closed, and permits the flow of the exhaust gas when the waste gate valve is opened.
As a technique for diagnosing presence or absence of abnormality, such as fixing of the waste gate valve, a technique described in Japanese Patent Application Publication No. 09-004507 (JP 09-004507 A) has conventionally been known. In JP 09-004507 A, supercharging pressure during supercharging and supercharging pressure during non-supercharging are compared, and presence of fixing is determined when a difference in the supercharging pressure is smaller than a value predicted in a normal time. In this way, the presence or absence of fixing of the waste gate valve is diagnosed.
In addition, as the supercharging system, a supercharger that is operated by power of the internal combustion engine for supercharging is also available. Then, this type of supercharging system that includes a relief valve in a portion of the intake passage on a downstream side of the supercharger, opens the relief valve when the supercharging pressure becomes excessively high, and lets some of the intake air flow out thereof to lower the supercharging pressure.
By the way, one type of abnormality of the waste gate valve is called slightly opened fixing, that is, the valve is fixed at a position slightly opened from a fully closed position and thus is not fully closed. Also during such slightly opened fixing, the supercharging pressure is increased to some extent. Thus, with the above conventional diagnostic method, which is based on the supercharging pressure, it may be impossible to detect the abnormality with sufficient accuracy.
In addition, there is a case where the relief valve of the above supercharger is slightly opened and fixed. Also during such slightly opened fixing of the relief valve, the supercharging pressure is increased to some extent. Thus, with the above conventional diagnostic method, which is based on the supercharging pressure, it is difficult to secure the diagnostic accuracy.